Active Salt/Silica-Templated 2D Mesoporous FeCo-Nx-Carbon as Bifunctional Oxygen Electrodes for Zinc–Air Batteries

Article abstract of DOI:10.1002/anie.201710852

Two types of templates, an active metal salt and silica nanoparticles, are used concurrently to achieve the facile synthesis of hierarchical meso/microporous FeCo-N_x-carbon nanosheets (meso/micro-FeCo-N_x-CN) with highly dispersed metal sites. The resulting meso/micro-FeCo-N_x-CN shows high and reversible oxygen electrocatalytic performances for both ORR and OER, thus having potential for applications in rechargeable Zn–air battery. Our approach creates a new pathway to fabricate 2D meso/microporous structured carbon architectures for bifunctional oxygen electrodes in rechargeable Zn–air battery as well as opens avenues to the scale-up production of rationally designed heteroatom-doped catalytic materials for a broad range of applications.

Full text of DOI:10.1002/anie.201710852

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Active Salt/Silica-Templated 2D Mesoporous FeCo-Nx-Carbon as
Bifunctional Oxygen-Electrodes for Zn-Air Batteries
Authors: Shuang Li, Chong Chen, Xiaojia Zhao, Johannes Schmidt,
and Arne Thomas
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710852
Angew. Chem. 10.1002/ange.201710852
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Active Salt/Silica-Templated 2D Mesoporous FeCo-Nx-Carbon as
Bi-functional Oxygen-Electrodes for Zn-Air Batteries
Shuang Li,[a] Chong Cheng,[b] Xiaojia Zhao,[a] Johannes Schmidt,[a] and Arne Thomas*[a]
Abstract: Heteroatom-doped carbons have received increasing
interest in constructing next-generation reversible oxygen
electrocatalysts for rechargeable Zn-air batteries. Even though many
types of heteroatom- doped carbons have been reported, the
rational design of heteroatom doped carbons with control of micro-
and nanostructure, porosity and surface area, amount and
dispersion of heteroatom and metal doping remains an enormous
challenge. Here, two types of templates, an active salt and silica
nanoparticles, are used concurrently to achieve the facile synthesis
of hierarchical meso- microporous FeCo-Nx-carbon nanosheets
(meso/micro-FeCo-Nx-CN) with highly dispersed metal sites. The
resulting meso/micro-FeCo-Nx-CN shows high and reversible
oxygen electrocatalytic performances for both ORR and OER, thus
having potential for applications in rechargeable Zn-air battery. Our
approach creates a new pathway to fabricate 2D meso/micro-
porous-structured carbon architectures for bifunctional oxygen
electrodes in rechargeable Zn-air battery as well as opens avenues
to the scale-up production of rationally designed heteroatom-doped
catalytic materials for a broad range of applications.
Recent studies demonstrated that heteroatom-doped carbon
nanomaterials are promising materials for bifunctional oxygen
catalysts.[4] For example, a metal-free, N, P-doped carbon
prepared by pyrolysis of a phytic acid containing polyaniline
aerogel was described as bi-functional ORR/OER catalyst in Zn-
air battery.[5] The catalyst showed a high ORR activity in term of
a half-wave-potential of 0.85 V vs. RHE. However, the OER
activity of such N, P-doped carbon metal free catalysts was less
significant, resulting in an Egap of ~1.0 V, while the Egap indicates
the potential difference between ORR half-wave-potential and
OER potential at 10 mA/cm2. To improve the OER performance,
metal-Nx-carbon catalysts were widely investigated.[6] Zhang et
al. reported a graphene based Co-Nx-C catalyst, which exhibited
good ORR/OER bi-functional activities yielding an Egap of ~0.95
V.[6a] Notably, while Co-Nx-doped carbons showed improved
OER activity, this is paid-off by an inferior ORR activity.
Therefore,
to
achieve
high-performance
bi-functional
electrochemical catalyst, synergetic doping of a highly ORR-
active and OER-active structures, such as bimetal Fe, Co, Ni,
Mn or Zn-Nx centers in one carbon catalysts has been proposed
and
investigated.[7]
However,
even
though
several
Rechargeable metal-air batteries are considered as one of the
most promising candidates as power source for next generation
electric vehicles and many other electrical devices because of
their low cost and high energy density.[1] The efficiency of
charge-discharge processes of metal-air batteries is significantly
affected by the electrocatalysts for the oxygen reduction reaction
(ORR) and oxygen evolution reaction (OER), respectively. At
present, one of the major challenges impeding the widespread
commercialization of rechargeable metal-air batteries reside in
the low onset and half-wave potential of ORR (i.e. determining
the working discharge voltage of the battery) and high
overpotential of OER (i.e. the charge voltage of the battery).[2]
Precious metal based electrocatalysts (e.g. Pt/C, RuO2, Ir/C))
usually possess high catalytic activity either for ORR or OER.
However, none of these commercial catalysts can satisfyingly
perform both reactions. Furthermore, poor long-term durability
caused by structural degradation or poisoning of catalytic
centers during electrochemical operations also hinder their
widespread applications.[3] Therefore, the development of highly
efficient and durable non-precious metal based catalysts, which
are simultaneously active for ORR and OER as bi-functional
electrodes to accelerate the charge and discharge processes is
inevitable for designing advanced rechargeable metal-air
batteries, however remains a major challenge.
metal/heteroatom-doped carbons have been reported for
reversible oxygen electrocatalysts, just few reports rationally
discussed how nanoarchitecture, such as morphology and
meso/micro-porosity of the carbon materials influence the
reversible oxygen catalytic activity and stability.[3a, 8]
Owing to their unique properties compared with conventional
porous carbon analogues, two-dimensional (2D) porous carbon
nanosheets, are attracting increasing attention for their potential
application in the fields of electronics, sensing, energy storage,
and conversion,[3b, 9] especially because the 2D morphology with
additional porosity increases the accessible surface area for the
electrolyte, thus accelerating mass and charge transfer.[3b, 8b, 9a,
10] Recently, several synthetic methods for the preparation of 2D
mesoporous carbon nanosheets have been reported, including
hard and soft templating approaches using silica nanoparticles
or amphiphilic block-co-polymers, respectively.[9b, 11] However, a
facile one-step method to produce not only 2D
meso/microporous, but also bimetal-Nx-doped carbons, which
can be applied as reversible ORR-OER catalysts has not been
reported. This is possibly due to the lack of suitable and easily-
scalable structure-directing agents to guide the assembly of 2D
morphologies as well as the inner-connected bimetal-Nx
meso/micro-structures.
Here, we report the synergistic utilization of two types of
templates, an active salt and silica nanoparticles, to achieve the
facile synthesis of 2D meso/microporous Fe/Co-Nx-doped
carbon nanosheets with excellent reversible ORR-OER catalytic
performance. In our approach, layered FeCl3·6H2O inorganic
crystal are applied as 2D active salt template, which first
coordinates with dopamine hydrochloride (DA) to form a layered
organic-inorganic hybrid structure (Figure 1a).[9a] Subsequently,
a certain amount of CoCl2·6H2O was added to enable Co doping.
Furthermore, silica nanoparticles (Ludox AS-40, particle size of
~22 nm) were mixed with this paste by vigorous grinding until a
[a]
Shuang Li, Xiaojia Zhao, Dr. Johannes Schmidt, Prof. Dr. Arne
Thomas
Functional Materials,
Department of Chemistry, Technische Universität Berlin
Hardenbergstr. 40, 10623, Berlin, Germany
E-mail: arne.thomas@tu-berlin.de
Dr. Chong Cheng
[b]
Chemistry and Biochemistry
Freie Universität Berlin Institution
Takustrasse 3, 14195, Berlin, Germany
Supporting information for this article is given via a link at the end of
the document.
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homogenous mixture is observed (Figure S1). The resulting DA-
metal salt-silica nanoparticle mixtures were thermally treated
under an inert atmosphere to yield FeCo-Nx-doped carbons with
encapsulated silica nanoparticles. The residual metal salts and
silica templates were then removed by etching and washing,
then a secondary thermal treatment was applied to yield the final
products named meso/micro-FeCo-Nx-CN. The amount of silica
templates was varied to tailor the amount of mesoporosity in the
carbon nanosheets. For comparison, carbon nanosheets without
addition of silica nanoparticles were also prepared to yield
entirely microporous materials (micro-FeCo-Nx-CN) (see details
in Table S1). All the meso/micro-FeCo-Nx-CN catalysts
discussed in the following part are synthesized with 30 wt.%
silica (meso/micro-FeCo-Nx-CN-30), if not noted otherwise.
Figure 1. a) Schematic illustration of the process of active salt/silica-templated 2D meso/micro-FeCo-Nx-CN-30; b, c) SEM, d) TEM and e) HAADF-STEM images
and the corresponding element mapping of C, N, Fe, and Co and f) high resolution HAADF-STEM images of meso/micro-FeCo-Nx-CN-30.
The meso/micro-FeCo-Nx-CN-30 catalysts were investigated by
scanning electron microscopy (SEM) (Figure 1). While micro-
FeCo-Nx-CN synthesized without silica NP template, shows a
uniform 2D nanosheet morphology (Figure S2) with smooth
surface, addition of silica template (Figure 1 b-c) yield a
pronounced mesoporous structure within the nanosheets. By
varying the amount of silica template, the porosity and thickness
of the pore walls can be adjusted. The morphology of
meso/micro-FeCo-Nx-CN prepared with 20% and 40% template
are shown in figure S3 and S4, both exhibiting a mesoporous
structure as well. When higher amounts of silica template are
applied (e.g. 40%), abundant mesopores are still visible, but the
structure seem to be partially collapsing, probably due to pore
wall thinning. (Figure S4). A silica amount of 30% thus seem to
be ideal to reach high mesoporsity and structural stability.
Atomic force microscopy (AFM) was conducted to verify the 2D
morphologies and the thickness of the nanosheets (Figure S5).
As shown in the AFM images, most of the as prepared micro-
FeCo-Nx-CN shows thickness ranged from 15.2-24.3 nm, and
the meso/micro-FeCo-Nx-CN shows a thickness of about 48.5
nm. Besides the increased thickness, the meso/micro-FeCo-Nx-
CN also shows increased surface roughness due to its
mesoporous structures.
To further identify the morphology and element distribution on
the mesoporous carbon nanosheets, transmission electron
microscopy (TEM), high angle annular dark-field scanning
transmission electron microscope (HAADF-STEM), high
resolution STEM (HR-STEM), and TEM elemental mapping were
measured. The TEM image in Figure 1d shows a thin nanosheet
with uniformly distributed mesoporous structures. The HAADF
TEM image (Figure 1e) and corresponding elemental mapping
of C, N, Fe, and Co reveals again the 2D morphology and the
uniform distribution of C, N, Fe, and Co elements within the
meso/micro-FeCo-Nx-CN nanosheets, indicating homogeneous
doping and distribution of active centers on the catalyst. Metal
distribution within the meso/micro-FeCo-Nx-CN were further
investigated by HAADF-STEM (Figure 1f). Within the pore walls,
very small isolated bright dots, with diameters of 0.1-0.2 nm can
be identified, attributable to single or multi-metal atoms doping in
the catalyst. It was reported that this kind of single or multi-
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atoms doping is favorable for the formation of Metal-Nx active
centers, therefore enhancing the performance of the catalyst.[12]
Figure 2. a) N2 adsorption/desorption isotherms, b) calculated mesoporous surface area from t-plot of the as prepared materials; c) pore size distribution curves
calculated from the adsorption branch of the isotherms by the QSDFT model.
The porous characteristics for the materials were further
analyzed by nitrogen sorption measurements. The calculated
Brunauer-Emmett-Teller (BET) surface areas of micro-FeCo-Nx-
CN and meso/micro-FeCo-Nx-CN-20, 30 and 40 are 1025, 1120,
1168 and 1321 m2/g, respectively. The N2 adsorption/desorption
measurements (Figure 2a) for micro-FeCo-Nx-CN shows a type I
isotherm, i.e. an explicitly microporous pore structure. For
meso/micro-FeCo-Nx-CNs the isotherms change to type IV
showing that an additional mesoporosity was successfully
introduced by the silica template. The mesoporous surface area
of micro-FeCo-Nx-CN and meso/micro-FeCo-Nx-CN-20, 30, 40
calculated from the t-plot are 124, 459, 652 and 832 m2/g
(Figure 2b). These measurements show that addition of silica
template does not significantly change the surface area but
rather increase the fraction of meso- to micropores. The pore
size distribution was analyzed by the QSDFT model to calculate
the micropore size and mesopore size separately (Figure 2c).
Meso/micro-FeCo-Nx-CN-30 shows some amount of micropores
and mesopores, while for the latter a maximum pore width is
seen around 20 nm, reflecting the size of the silica nanoparticle
template. In contrast to micro-FeCo-Nx-CN very small pores in
the micropore region (<0.7 nm) have significantly decreased,
explaining the low change in overall surface area. Also for
meso/micro-FeCo-Nx-CN-20 abundant mesopores are visible,
again with a maximum at ~ 20 nm pore diameter, while for
meso/micro-FeCo-Nx-CN-40 a broader pore size distribution is
observed, pointing to the partial collapse of the mesopores
structure due to the thinning of the pore walls. (Figure S6).
X-ray photoelectron spectroscopy (XPS) reveals nitrogen
contents of 5.39 and 5.10 atomic % in micro-FeCo-Nx-CN and
meso/micro-FeCo-Nx-CN-30, respectively (Figure S7). The high-
resolution N 1s spectra of the catalysts show that addition of
silica template did not significantly change the chemical
structure of the material, since the ratio of different N species is
similar in all FeCo-Nx-CN catalysts. The metal content in
meso/micro-FeCo-Nx-CN is 0.52 atomic % (0.40 for iron, and
0.12 for cobalt), and in micro-FeCo-Nx-CN is 0.47 atomic %
(0.33 for iron, and 0.14 for cobalt) (Figure S8). The high
resolution XPS spectra of Fe and Co peaks (Figure S9) was
fitted into only Fe2+, Fe3+, Co2+, and Co3+ for both samples of
micro-FeCo-Nx-CN and meso/micro-FeCo-Nx-CN, no metallic Fe
and Co was detected, which were consistent with the powder X-
Ray Diffraction (XRD) results (Figure S10), thus indicating there
are only atomic dispersion of Fe and Co active site in the N-
doped carbon matrix, also known as Fe-Nx and Co-Nx.[12] The
similar elemental component from XPS, indicates that the
introducing of mesoporosity did not significantly change the
chemical structure and composition of the materials.
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Figure 3. a) Schematic illustration of ORR and OER on meso/micro-FeCo-Nx-CN. b) RDE polarization curves for micro-FeCo-Nx-CN, meso/micro-FeCo-Nx-CN,
meso/micro-Fe-Nx-CN and Pt/C catalysts for ORR. c) RDE polarization curves of micro-FeCo-Nx-CN, meso/micro-FeCo-Nx-CN, meso/micro-Fe-Nx-CN and Pt/C
catalysts for OER. d) Corresponding Tafel plot of OER activity in c. e and f) Egap difference of micro-FeCo-Nx-CN and meso/micro-FeCo-Nx-CN. Durability of the
meso/micro-FeCo-Nx-CN catalyst g) for ORR and f) for OER. h) Comparison of Egap values for our catalysts with some current designed state-of-art bifunctional
catalysts, these catalysts are cited from references 5-7.
It can be assumed that the meso/microporous structure
found in the silica templated material will be beneficial for both
gas and electrolyte diffusion during the electrochemical reaction
(Figure 3a). Therefore, the catalytic performance of meso/micro-
FeCo-Nx-CN and micro-FeCo-Nx-CN catalysts towards both
ORR and OER were evaluated and compared using the rotating
disk electrode (RDE) technique. The polarization curves for
meso/micro-FeCo-Nx-CN-30 catalysts for ORR (Figure 3b) show
a high onset potential of 954 mV vs. RHE and half-wave
potential of 886 mV vs. RHE, which are 54 mV and 58 mV
higher for micro-FeCo-Nx-CN. Furthermore, the limited current
OER curve (Figure 3d) reveals a low Tafel slope of 57 mV/dec
for meso/micro-FeCo-Nx-CN-30 suggesting good kinetic
a
process (which indicates the natural activity of the catalytic
active site). This remarkable ORR and OER bifunctional activity
of meso/micro-FeCo-Nx-CN gives an exceptional low Egap of
0.78V between ORR half-wave potential and OER potential at a
current density of 10 mA/cm2 (Figure 3e). Figure 3f shows the
Egap is decreased from 0.88V to 0.78V owning to the introducing
of mesoporosity. Meso/micro-FeCo-Nx-CN-20 and -40 were also
investigated for OER (Figure S9), which again reveal the
enhanced activity by introducing mesoporosity, except for
density for meso/micro-FeCo-Nx-CN-30 can reach to 6.3 mA/cm2, meso/micro-FeCo-Nx-CN-40 in which the partial structural
which is much higher than the one observed for micro-FeCo-Nx-
CN (5.3 mA/cm2) and even for a commercial Pt/C catalyst (5.6
mA/cm2). This high limited current density of meso/micro-FeCo-
collapse seems to hinder fast gas diffusion.
To further reveal the influence of the different metal species
on catalytic activity a comparable material but without Co
addition (meso/micro-Fe-Nx-CN-30) was synthesized and the
activity of for both ORR and OER were compared. Although
meso/micro-Fe-Nx-CN-30 shows almost the same onset
potential and half-wave potential for ORR with meso/micro-
Nx-CN catalysts seem indeed
a
consequence of the
hierarchically micro- and mesoporous structure, which facilitates
gas diffusion and mass transfer. Indeed, also all other
meso/micro-FeCo-Nx-CN catalysts (Figure S8) synthesized with
different ratio of silica template show higher onset potential, half-
wave potential and limited current density than micro-FeCo-Nx-
CN. The OER polarization curves in Figure 3c shows that the
onset of meso/micro-FeCo-Nx-CN-30 (1.60 V) is lower than for
micro-FeCo-Nx-CN (1.63 V), also the potential at current density
of 10 mA/cm2 is 1.67 V for meso/micro-FeCo-Nx-CN-30 and 1.71
V for micro-FeCo-Nx-CN. The corresponding Tafel plot of the
FeCo-Nx-CN,
a significant difference in OER reaction is
observed by introducing Co. The OER potential at a current
density of 10 mA/cm2 for meso/micro-Fe-Nx-CN is 1.71V, i.e.
0.04V higher than for meso/micro-FeCo-Nx-CN. The Tafel plot of
meso/micro-Fe-Nx-CN shows a Tafel slope of 102 mV/dec which
is again much higher than that for meso/micro-FeCo-Nx-CN (57
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mV/dec) indicating is much slower kinetic process. It can thus be
concluding that the activity of the FeCo-Nx active sites is much
higher than for a Fe-Nx active site alone. Therefore, the stability
of the meso/micro-FeCo-Nx-CN-30 for both ORR and OER were
measured by chronoamperometry scan (Figure 3g) and
chronopotentiometry scan (Figure 3h). The current of ORR after
10 hours can maintain more than 95%. When keep working at
current density of 10 mA/cm2 for OER, the potential can
maintain under 1.7 V. These show good durability of
meso/micro-FeCo-Nx-CN-30 for both ORR and OER during
longtime work. Furthermore, we also summarized and compared
the performances of some current designed state-of-art
bifunctional catalysts with our meso/micro-FeCo-Nx-30 catalyst
in terms of the Egap and the catalyst loading amounts (Figure 3h),
more detailed data comparison was shown in Table S2. Based
on these analysis, we can draw the conclusion that our catalyst
shows outstanding reversible oxygen catalytic performances.
Figure 4. a) Open-circuit-potential of the Zn-air battery equipped with meso/micro-FeCo-Nx-CN-30 catalyst; b) cycle performance of Zn-air batteries with
meso/micro-FeCo-Nx-CN-30 and Pt/C catalysts at a current density of 5 mA/cm2 and then 10 mA/cm2 (20 mins for each cycle); c) cycle performance of
meso/micro-FeCo-Nx-CN-30 equipped battery at a current density of 20 mA/cm2 for 20 hours; d) discharge curves of Zn-air batteries with meso/micro-FeCo-Nx-
CN-30 and Pt/C catalysts and the corresponding power density; e) long-time charge and discharge of the meso/micro-FeCo-Nx-CN-30 equipped battery at 10
mA/cm2 (2 hours for each cycle).
A rechargeable liquid Zn-air battery was assembled to
evaluate the activity of the bifunctional meso/micro-FeCo-Nx-CN
catalysts for such devices (Figure S13). A Zn foil was used as
anode and the catalyst-loaded carbon cloth with gas diffusion
layer as cathode. 0.2 M Zn(OAc)2 in 6.0 M aqueous KOH was
used as electrolyte. As shown in Figure 4a, the open-circuit-
potential of the Zn-air battery equipped with meso/micro-FeCo-
Nx-CN is ~1.4 V. The discharge curves and corresponding
power density of the Zn-air battery catalyzed by meso/micro-
FeCo-Nx-CN in comparison with Pt/C, suggesting the superior
activity of meso/micro-FeCo-Nx-CN. The maximum power
density of meso/micro-FeCo-Nx-CN battery is determined to be
as high as 150 mW/cm2, three times higher than the power
density of Pt/C. The rechargeability of the batteries were
measured by charge and discharge cycles under different
current density (Figure 4b and 4c). When charge-discharge
cycling test at current density of 5 mA/cm2 (Figure 4b) were
carried out, the discharge voltage of the meso/micro-FeCo-Nx-
CN equipped battery is about 1.26 V, 0.1 V higher than that of
Pt/C equipped battery (1.16 V); the charge voltage is 2.01 V,
even 0.5 V lower than that of Pt/C equipped battery (2.51 V).
When the cycling test is carried out more than 40 hours, the
voltage of the meso/micro-FeCo-Nx-CN equipped battery is still
quite stable, while Pt/C based battery shows a large voltage
decrease in the discharge process and also large voltage
increase during the charge process. The performance and
stability of meso/micro-FeCo-Nx-CN equipped battery at higher
charge-discharge current density was also detected, showing
that the battery run stable for more than 20 hours under 20
mA/cm2, the charge-discharge voltage gap in this case was only
~0.91 V. An excellent cyclability of the battery can be observed
even at long charge-discharge periods (2 hours), indicating the
high application potential in rechargeable Zn-air battery. The
above results highlight again the high ORR and OER catalytic
activity and stability of the meso/micro-FeCo-Nx-CN catalysts,
which can work robustly as bifunctional catalysts for advanced
rechargeable Zn-air electrode and other related applications.
The performance of our Zn-air battery is also compared with the
currently reported results in Table S3 for detail. The superior
electrochemical performances of meso/micro-FeCo-Nx-CN
catalysts for both ORR and OER can be attributed to several
factors: (1) the nanometer thickness of carbon sheets can
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efficiently enhance the transfer speed for both electrons and
oxygen; (2) the combination of Fe, Co with N-doping can
modulate the electronic properties and surface polarities, thus
improving the activity of the catalyst; (3) the high specific surface
area and balanced microporous and mesoporous structures
make the active sites fully accessible and improve the transport
properties of ORR/OER-relevant species.
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materials exhibit with
a
well-defined 2D morphology,
hierarchically meso- and microporous structure, homogenous
dispersion of single metal-atoms or very small metal-clusters
and enriched reversible oxygen electrocatalytic sites. These
structural features eventually guarantee its excellent
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Keywords: active salt/silica-templating • 2D meso/micro-
structured carbon • FeCo-Nx heteroatoms doping • reversible
oxygen electrocatalyst • rechargeable Zn-air battery
This article is protected by copyright. All rights reserved.

10.1002/anie.201710852
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
An active salt/silica-templating
approach for the facile synthesis of
hierarchical meso/microporous
Fe/Co-Nx-doped carbon nanosheets
is presented. These materials achieve
high performance as bi-functional
oxygen-electrodes. The resulting
FeCo-Nx-carbon reveals a well-
defined 2D morphology and
Shuang Li, Chong Cheng, Xiaojia Zhao,
Johannes Schmidt, and Arne Thomas*
Page No. – Page No.
Active Salt/Silica-Templated 2D
Mesoporous FeCo-Nx-Carbon as Bi-
functional Oxygen-Electrodes for Zn-
Air Batteries
homogenous metal-atom doping,
which yield enriched active sites for
ORR-OER. These unique features
lead to excellent and reversible
oxygen electrocatalytic performance,
which show its potential in
rechargeable Zn-air battery.
This article is protected by copyright. All rights reserved.